CORE-16323: Improve State Manager Index

Update recently introduced index so it uses both the "key" and
"version" columns as both are used by the State Manager during update
and delete queries.

data/db-schema/src/main/resources/net/corda/db/schema/statemanager/migration/state-manager-migration-v5.1.xml

@@ -30,7 +30,8 @@
                 <constraints nullable="false"/>
             </column>
         </createTable>
-        <createIndex tableName="state" indexName="state_version_idx">
+        <createIndex tableName="state" indexName="state_key_version_idx">
+            <column name="key"/>
             <column name="version"/>
         </createIndex>
         <addPrimaryKey columnNames="key" constraintName="state_key" tableName="state"/>